Although much studied, the relationships
between water chemistry and floristic
composition of mire vegetation are still imperfectly understood. Acidity/alkalinity
levels are known to be of great importance and are most easily measured as pH
and calcium carbonate content. The oxygen content and level of oxidising or
reducing conditions of mire waters are also important, and various chemical factors
are influenced by and interact with physical conditions such as rate of water
movement and amplitude of water table fluctuation. The physical and chemical
characteristics of the peat itself are important, and though they are essentially
dependent on the water regime, they may affect it, as by buffering effects and by
drawing up the water table. It is perhaps best to think of the peat and mire water as
a single system with complex cause/effect relationships which involve some degree
of circularity.
Acidity/alkalinity has been much used
as an index of nutrient status of waters, and
the terms oligotrophic, meso-trophic and eutrophic have become generally
accepted terms for different nutrient levels (their original meaning was not quite the
same), though there is no general agreement about their quantitative definition. In
the present account they are defined and applied as follows on p. 251.
The nutrient requirements of individual
plant species and communities may be
described in terms of these three levels, which thus give a simple but useful means
of analysing and classifying mire vegetation in one direction. A number of mire
species have a very wide range of tolerance/ requirements in regard to nutrient
status, and so cannot be regarded as diagnostic of any particular trophic level of
mire vegetation. They include Carex rostrata, C. nigra, C. limosa, C. lasiocarpa,
Eriophorum angustifolium, Juncus acutiflorus, Menyanthes trifoliata and Myrica
gale. A few species show marked regional change in adaptation to nutrient status,
being strongly basiphilous in the south and east of Britain, but much less so or even
indifferent in the north and west, e.g. Cladium mariscus, Schoenus nigricans,
Drosera anglica, Phragmites communis, Carex dioica, Utricularia vulgaris,
Scorpidium scorpioides and Drepanocladus revolvens.
The pH values of peat tend to be lower
than those of mire water, by up to one unit,
and the more acidic peats are prone to seasonal variation in pH, as the greater
amount of oxidation under drier, summer conditions increases the level of acidity.
